Field of the Invention
The invention concerns a method and a pulse sequence determination device to determine a pulse sequence for a magnetic resonance system. Moreover, the invention concerns a method to operate a magnetic resonance system using such a pulse sequence, as well as a magnetic resonance system with a radio-frequency transmission device that is designed in order to emit a radio-frequency pulse train to implement a desired measurement on the basis of a predetermined pulse sequence, and to emit a gradient pulse train via the gradient system, in coordination with the emission of the radio-frequency pulse train.
Description of the Prior Art
In a magnetic resonance system also called a magnetic resonance tomography system, the body to be examined is typically exposed to a relatively high basic magnetic field (for example of 1, 5, 3 or 7 Tesla) with the use of a basic field magnet system. A magnetic field gradient is additionally applied by a gradient system. By means of suitable antenna devices, radio-frequency excitation signals (RF signals) are then emitted via a radio-frequency transmission system. The radio-frequency excitation signals are designed to cause the nuclear spins of specific atoms to be excited to resonance by the radio-frequency field, so as to be flipped (deflected) by a defined flip angle relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, radio-frequency signals known as magnetic resonance signals, are radiated by the excited nuclear spins, and are received by means of suitable reception antennas and then processed further. Finally, the desired image data can be reconstructed from the raw data acquired in such a manner.
For a specific measurement, a pulse sequence is to be emitted with a radio-frequency pulse train to be emitted and a gradient pulse train to be switched in coordination with this (with matching gradient pulses in the slice selection direction, in the phase coding direction and in the readout direction, frequently in the x-direction, y-direction and z-direction). In particular, the timing within the sequence—i.e. at which time intervals which pulses follow one another—is thereby significant to the imaging. A number of control parameter values is normally defined in what is known as a measurement protocol, which is created beforehand and can be retrieved (for example from a memory) for a specific measurement and can possibly be modified by the operator on site, which operator can provide additional control parameter values (for example a defined slice interval of a stack of slices to be measured, a slice thickness etc.). A pulse sequence is then calculated on the basis of all of these control parameter values, which pulse sequence is also designated as a measurement sequence, MR sequence (magnetic resonance sequence), or shortened to just sequence.
The readout processes of the magnetic resonance signals—i.e. the acquisition of raw data—are defined just like the emission of the radio-frequency signals in what is known as “k-space”. Arbitrary points in k-space can be approached via appropriate switching (activation) of the gradients. K-space is the positional frequency domain, and a trajectory in k-space (also called a “k-space trajectory” or shortened to “trajectory” in the following) describes the path for the entry of data into k-space that is chronologically traversed given emission of an RF pulse or given readout via corresponding switching of the gradient pulses. During a resonance measurement, k-space is filled with raw data by traveling defined k-space trajectories during the raw data acquisition, and the image data are then reconstructed from these raw data via a Fourier transformation.
In order to fill k-space, different patterns are traversed—for example Cartesian patterns, in which individual routes of the k-space trajectory are traveled line-by-line (for example), but also spoke-like or spiral-shaped patterns. This depends on the respective sequence type, among other things.
Such pulse sequences are typically crated by special sequence programmers. The creation is thereby based on the precise definition or, respectively, implementation of the individual gradient curves, wherein the precise timing and the shape and strength of the individual gradient pulses is provided by the sequence programmer depending on the sequence type. It has been necessary for the programming of the sequence to conventionally take place very close to hardware, meaning that it is dependent on the respective type of the magnetic resonance system on which the MR sequence should run. This method is therefore relatively complicated, and requires highly specialized programmers and normally special, apparatus-dependent programming tools.